Packaging materials used for packaging food products, pharmaceutical products, and the like are desired to prevent deterioration of the contents. For example, it is desired that packaging materials for food products are able to minimize oxidation and deterioration of protein, oils, fats, and the like and further preserve the flavor and freshness. Also, it is desired that packaging materials for pharmaceutical products, which must be handled under aseptic conditions, are able to minimize deterioration of the active ingredients in the contents and retain the efficacy of the active ingredients.
Such deterioration of contents is mainly caused by oxygen or water vapor transmitted through the packaging material or other gases that react with the contents. Therefore, packaging materials used for packaging food products, pharmaceutical products, and the like are desired to have properties of not transmitting gases, such as oxygen and water vapor, therethrough (gas barrier properties).
To meet such requests, there have been used conventionally gas barrier films configured with polymers having relatively high gas barrier properties (gas barrier polymers), or laminates (laminated films) using gas barrier films as base films.
Conventionally, polymers containing a highly hydrophilic hydrogen bonding group in the molecules, as represented by poly(meth)acrylic acid or polyvinyl alcohol, have been used as gas barrier polymers. Packaging materials made of such polymers exhibit very good gas barrier properties against oxygen and the like under dry conditions. However, the packaging materials made of such polymers have suffered from problems of their gas barrier properties against oxygen and the like being greatly impaired under highly humid conditions due to the hydrophilicity of the hydrogen bonding group, and being poor in resistance to humidity and hot water.
To solve such problems, it is known that a polycarboxylic acid polymer layer and a polyvalent metal compound-containing layer, which are permitted to be adjacent to each other, are laminated onto a substrate, and a reaction is provoked between the two layers to produce a polyvalent metal salt of the polycarboxylic acid polymer, thereby providing a gas barrier packaging material (e.g., refer to PTLs 1 and 2). According to the disclosure, the gas barrier packaging material thus obtained has high oxygen gas barrier properties even under high humidity conditions.
Further, there is known a gas barrier packaging material in which the oxygen gas barrier properties do not have a dependence on humidity. In this case, by applying retort treatment, the gas barrier packaging material can be spared the need for expressing the oxygen gas barrier properties (e.g., refer to PTLs 3 to 5).